Yasumasa Tsuji

Force-free magnetic field in the axisymmetric torus of arbitrary aspect ratio

The equation ∇×B=αB is solved analytically in a toroidal coordinates system for the axisymmetric torus of arbitrary aspect ratio without the approximation of large aspect ratios.The solution is derived from the Helmholtz equation and is expressed in the series that contains the associated Legendre functions. A set of fundamental solutions is presented, each of which approaches asymptotically the solution for cylindrical geometry, whose azimuthal dependence is cos lψ, in the limit of large aspect ratios. The force‐free magnetic field is given in the form of the infinite series of these solutions. By using the set of fundamental solutions, the contour of the magnetic surface, eigenvalues, and F‐Θ curves of the circular cross section are calculated.

Population inversion between the ground and first excited states in a recombining hydrogen plasma

Abstract Plasma conditions for generating a population inversion between the ground and first excited states in a recombining hydrogen plasma have been investigated on the basis of the CR model. Population inversion can be expected when three-body recombination plays a dominant role; the required regions of electron density and temperature are specified. It is shown that upper bounds exist for the ground-state population density at given electron density and temperature. Larger inversion densities can be obtained between the ground and first excited states than between excited levels. Numerical results are presented.

Initial Condition and Cooling Time of Recombining Helium Plasma for Stationary VUV Lasing Action

Calculated optimum conditions as to initial electron density and cooling rate of a recombining helium plasma for the production of VUV laser oscillation of the Hell 164 nm line are presented together with an explanation of the population inversion mechanism. For high initial plasma density (~1018 cm-3) rapid cooling (τE ≦10-13 sec) should produce a short pulse laser oscillation. For medium initial density (~ 10-16 cm-3), stationary laser oscillation can be produced by much slower cooling (τE ~ 10-8 sec) but on cooling with τE > 10-7 sec the oscillation can hardly be realized. For lower initial plasma density ( ≦ 1015 cm-3), laser oscillation is not possible because the population inversion obtained is not sufficient.